Method and apparatus for removing the stresses from plastic sheeting



L. YOUNG s Sheets-Sheet 1 May 25, 1943. mmaon AND APPARATUS FOR nnmovme THE STRESSES FROM PLASTIC SHEETING Filed June 20, 1940 May 25, 1943. H. L. YOUNG 2,320,115

METHOD AND APPARATUS FOR REMOVING THE STRESSES FROM PLASTIC SHEETING Filed June-20, 1940 3 Sheets-Sheet 2 3% JrT/sn 0a y 1943. H. YOUNG 2,320,115

METHOD AND APPARATUS FOR REMOVING THE STRESSES FROM PLASTIC SHEETING Filed June 20, 1940 3 Sheets-Sheet. 3

ing the stresses from plastic sheeting.

Patented May 25, 1943 Mc'rno'n AND umnsrus non aaMovmc 'rnr: s'racssas mom ms'rrc snaa'rmc Harvey-L. Young'Akron, Ohio, asslgnor to The B. F. Goodrich Company, New York, N. Y., a corporation of New York I I Application June 20, 1940, Serial No. 341,505

7 Claims.

This invention relates to apparatus for remov It is well known that plastic materials'which are formed into continuous lengths by calendering or extrusion ordinarily contain residual internal stresses. These stresses are generally believed to be caused at least in part by the fact that the plastic is of necessity deformed in the shaping process so that at the instant it leaves the calender rolls or die, it is considerably stressed. The sudden chillingby contact with v the atmosphere prevents or greatly retards further change in shape. [The rmulting plastic sheeting has a constant tendency to relieve its internal stresses by shrinking longitudinally andbecoming wider and thicker, which tendency is ordinarily unnoticeable at room temperatures but becomes very pronounced when the sheeting is heated. Plastic sheeting containing residual calender grain also exhibits an abnormally small longitudinal extensibility and tear resistance.

Prior expedients for shrinking plastic sheet material and removing the internal stresses therefrom have been unsatisfactory for a number of reasons. When the sheeting has beenpermitted to shrink upon a conveyor or other rigid support, the friction of the sheeting therewith as shrinking occurred has either tom the sheeting or caused it to wrinkle and buckle, thereby introducing additional irregularities. The sheeting must ordinarily be heated to .a temperature at which it is very nearly fluid to remove completely the internal stresses, and many difliculties have been encountered due to adhesion-of the plastic sheeting tothe supporting means at this tem-' perature. It has accordingly proveri nearly imin frictional contact with a wheel. In the preferred embodiment of the invention, however, a fluid drive is employed inside the conveying rolls in such ,a manner that the passage of fluid through the rolls both propels and cools the same as hereafter described.

f the accompanying drawings: Fig. 1 is a side elevation of apparatus embodying my invention in its preferred form.

Fig. 2 is 'a' plan view of the apparatus shown inFig. 1.

Fig. 8 is asection on an enlarged scale taken ,on line 3-3 of Fig. 1.

possible to produce a uniform, smooth sheeting containing no internals'tressesby using'apparatus hitherto available. 1 1

The chief objects of my invention are to provide means for supporting the sheeting during to the supply lines 25, 25 and .26, 25 for the indishrinkage without anyfrictio'nal drag between the supports and thefsheeting; to provide means for conveying thesheetingthrough the heating zone without subjecting the sheeting to any tension; and to permit the simultaneous longitudinal shrinkage and latitudinal expansion of the sheeting without introducing any irregularities. I have found that these and other objects may ,beaccomplished by conveying plastic sheet material through a heated oven on a number of rolls which also serve to support the material. These rolls are driven by a frictional driving means'so arranged and constructed that when the sheet material shrinks, slippage occurs lie-'- tweenthe driving means and the rollrather than between the roll and the sheet material. This frictional driving means may include any nonpositive drive in which slippage may occur such as a positively-driven disc with a rough surface Fig. 2.

. Fig. 41s a still'further enlarged side elevation of a portion of Fig. 1.

Fig. 5, is an enlarged section taken on line 55 of Fig. 2.

' Fig-6,is anenlarged section-taken on line 6-6 of Fig. 3.

Fig. ,7'is an enlarged plan view of a portion of h of Fig. 3; and

Fig. 9 is an enlarged section taken on line 9-.9 of Fig. 3.

Referring to the drawings, the apparatus of this invention consists of an oven Ill lined with suitable insulating material ii and supported upon a frameconsisting of upright supports I! connected by horizontal frame members H. The oven 10 is provided with a hood l5 ers I6, i8 in the exhaust pipes I1, I! leading into the larger exhaust pipe i8 which carries away any fumes. v

The oven ill is heated by a series of gas burners 20 and 2|, shown in Fig. 3, in the bottom and top of the oven, supplied with gas by the maingas line 22 which divides into lines 23 and 24 leading vidual burners in the oven l0. Other heating imeansssuch as-radiant electric heaters may be employed in the oven if desired.

Extending across and projecting through the walls of the oven iii are a number of tubular conveying rolls 30 which are spaced at intervals along the length of the oven iii to support and convey the plastic sheet through 'the heating zone. These rolls 30 are preferably made of light -rnetal such as aluminum or an,aluminum alloy,

and are rotated :by a frictional drive adjusted so that when the plastic sheet shrinks, slippage occursbetween the frictional-drive and the rolls rather than between the rolls and the plastic sheet;

shown, the rolls are rotated by a fluid drive which In the preferred embodiment herein serves the double function of cooling the rolls, whereby the sticking of the plastic material *to .the rolls is prevented, and of providing motive power which rotates the conveying rolls at the exact speed required-to move the shrinking plastic having damp-' through the oven. It will be understood that the variations in the amount of shrinkage of different portions of the plastic necessitate a convey ing means having a readily variable speed iif'the heated shrinking plastic is to be carried through the oven without imparting any stresses to the plastic. apparatus herein described is automatically adjusted at the proper rate because of the method by which the conveying rolls are driven.

As shown in Fig. 8, the tubular conveying rolls- 30 are provided at each end of the portion outside the oven I with a steel bushing 3| and are supported on both sides of the oven l0 by antifriction bearings 32. The rolls are driven by compressed air which is passed from the supply line 33 provided with the valve 34 through the pipe 35 which extends through the part of the conveying roll 30 inside the oven I0. As shown in Fig. 3, theair is directed through the jets 36 drilled in the pipe 35 tangential to the inner diameter. By this means, air is directed against the wall of' the conveyingtube 30, thereby cooling and rotating the same. In some cases, and particularly where rather thick sheets of plastic materials are being shrunk, the air issuing through the jets 36 may be insufiicient to rotate the conveying rolls 30. In this case, as many of the rolls as necessary may be provided with simple air turbines such as that shownin Fig. 9 consisting of a jet 40 through which air may be passed and impinge onthe teeth of the gear 4i attached to the conveyingroll 30. Greater power may also be provided by placing vanes on the inside walls of .the conveying rolls 30- if. desired. Although it The speed of each conveying roll in the wrinkles from the plastic sheet 53 before it enters the oven I0.

The plastic sheet 53 isthenpassed under roll 10, over roll 1 I and through a slit in the end of the oven I0, and is carried through the oven by the conveying rolls 30. The immediate response of the rolls tochanges in length of the plastic sheet 53 manifested by changes in speed of the rolls permits the plastic sheet to pass through the oven without being subjected to any substantial frictional drag, and allows the sheet to shrink freely in any direction necessary for the removal 'of in tank 16in which it is cooled by a spray .of

' water issuing from the pipe 11. It then passes under guide roll I8, over roll I9 and dropsv to' conveyor belt 80, driven by roll 8|, which carries it to a point over second tank .82.-. Endless conveyorbelt 80 also passes around roll "I2, and

' may be tightened or loosened by-movlng roll 'II has been found most convenient to employ air to drive the, conveying rolls 30, other fluids such as water may be employed with the necessary changes in design. a

A number of spreading devices to keep the plastic sheet free of longitudinal wrinkles while shrinking are also placed inside the oven at appropriate intervals. I .The preferred spreading devices are tentering rolls shown in detail in Figs. j 5 and '7. A number of hollow rolls are placed on an arcuate tube 46 supported byjthe frame'4'I.

v The tube 46 is connected to a somewhat flexible air supply tube 48, and warm air is blown through the tube 46 while the apparatus is in operation to maintain the tentering rolls 45 at a tempera-. n

ture somewhat below. that of the oven to prevent the plastic from sticking to the 'rolls. Although i it is'not ordinarily necessary, the tentering rolls 45 may also serve as conveying rolls if jets are provided in the tube 46 to allow the escape of air inside the tentering rolls 45.

The spreading devices may be raised or lowered by rotating'the rod threaded to the boss 5| on the frame 41 and journaled in the bracket 49 attached to the frame I3. The rod 50 is provided with a squared end .52 to facilitate application of a tool to rotate the shaft.

At one end of the apparatus, a shaft 54 containing a roll of plastic sheeting 53 is mounted in a slotted bracket 56 attached to the frame member I 3, and is held in place by a pin 5'! removably mounted in the entrance to the slot:in the bracket 55. tively passed between presser roll 58 and driven roll 59, and presser roll 60 and driven roll 6|. Presser rolls 58 and 60 are mounted in yokes 62 and 63 which are pivotally mounted in the frame members I4, I 4, and provided with the handles 64 and 65. Ro1l"6| .is preferably driven at a slightly greater speed than rollv59 to remove any The plastic sheet 53 is then consecuand is wound upon shaft I02 toward roll 8| or roll I2. The sheet then drops into the tank 82 where it is again sprayed with water. from pipes 84 and 85. An'overfiow'pip'e 06 runs from the tank I6 to the tank .82 whi'ch; is provided with another overflow pipe'81. The

preliminarycooling in the first tank J6 prevents deformation of the'sheet during -itspassage to and through tank 82 where his cooled toroom' The sheet 53 is then dried, as shown in detail in Fig. 4, by passingthe sheet.

88 which directs} streams of air against both sides of 'thesheet from'jets 90- and ill in' pipes 92 and 93 connected to the air-supply pipe 94 and supported by the temperature.

through an air blast drier bracket 95 attached to the frame structure I3.

The plastic sheet 53 then passes over the driven roll'96, under the roll 91, .over tentering roll 89, under tentering roll I00, over tentering roll I M.

tively driven ,roll I03.

In the bottom of the oven are a number of doors I05, shown in Fig. 6, fastened to the oven I0 byhinges. I 06, and held in place by a=toothed handle." I01 which engages a slit in the bracket. I00 also attached to the oven I0. These doors areuse ful for removing from the oven any plastic material whichmay fall from the conveying rolls, andgnu'ay .be left slightly open for ventilation if desired'.'

The power mechanism for operating-the posi-- tively-driven rolls 59, 6|, 8 I, 96, and I03 comprises the motor I I0 connected to one end of the apparatus by sprocket chain III driving variablespeed transmission device II 2, which drives an-' other variable-speed transmissiondevice 3 by means of sprocket chain I I4, and by an additional chain H5 also drives roll 6I. Roll 59 is driventhrough the second variable-speed transmission .II3 by means of sprocket chain H6. The motor. I III) drives the other end of the apparatusby means of sprocketchain II'I driving an idler II8 from which roll BI is driven by chain H9, and

from which another chain I20 drives a variablespeed transmission'device I2I, which initurn drives roll I03 by means of a chain I22. Roll I03 in turn drives roll 96 by means of sprocket chain I I23, variable-speed transmission device I chain I25. fi

by means of posi By suitable adjustment or the variable-speed transmissions H2, I II, III and I24 the relative velocity of the various portions of the sheet 53 passing through the apparatus are so adjusted as to compensate for the shrinkage of the sheet and assure uniform progress of the sheet at all points of its travel without subjecting it to any substantial tension while it is softened by heat.

The apparatus herein described is very useful for shrinking calendered plastic organic polymers which often contain internal stresses of considerable magnitude. One such material is plasticized gamma polyvinyl chloride, which may be prepared by mixing at room temperature the follow ingingredients:

Parts by wt. Gamma polyvinyl chloride 58.49 Tricresyl phosphate (plasticizer) 36.50 Lead silicate (stabilizer) 1.94 Litharge (stabilizer) 2.91 Carbon black (stabilizer) .10

and homogenizing the mixture on a roll mill or the like for 13 min. at about 230 F., after which it is calendered at about 240 F. on a four-roll calender to form a sheet 33 inches wide and 0.011 inch thick. This plastic sheet is conveyed through the shrinking apparatus described in which the oven i is ten feet long at a rate of about fifteen feet per minute. i The end of the oven at which the plastic sheet 53 enters is main tained at about 550 F., the middle of the oven is maintained at about 450 F., and the temperature of the oven where plastic sheet leaves is about 350 F. While in the oven, the sheet shrinks longitudinally about 30%, increases in width to about 36 inches and in thickness to about .015 inch. Although the temperature of the plastic nearly reaches its melting point in the oven, the fact that there is practically no frictional drag upon the sheet by the conveying rolls permits shrinkage without tearing the sheet or introducing any new stresses therein.

While I have disclosed a specific embodiment of the apparatus constructed in accordance with the invention and well adapted to the practice of the shrinking process, it is to be understood that various structural changes may be made in the apparatus without departing from the spirit and scope of the invention as defined in the appended claims.

- material containing internal stresses comprising an oven, means for heating the oven to a temperature at which the plastic material softens and releases stresses, means for supporting and conveying the sheet material through the oven including a series of closely spaced rolls, and individual driving means for the rolls including a continuously operative frictional drive so constructed and arranged that when the sheet material shrinks, slippage occurs between the driving means and the rolls rather than between the rolls and the sheet material.

2. An apparatus for shrinking plastic sheet material containing internal stresses comprising an oven, means for heating the oven to a temperature at which the plastic material softens and releases stresses, means for conveying the sheet material through the oven including a series of rolls extending across the oven in spaced-apart relationship but sufllciently close to support the heat-softened sheet material, and individual, continuously operative fluid driving means for the rolls so constructed and arranged that when the sheet material shrinks, slippage occurs between thedriving means and the rolls rather than between the rolls and the sheet material.

3. An apparatus for shrinking plastic sheet material containing internal stresses comprising an oven, means for heating the oven to a temperature at which the plastic material softens and releases stresses, means for conveying the sheet material through the oven including a series of 'closely spaced rolls, means for spreading the sheet material and preventing longitudinal wrinkles when the sheet material increases in width, and individual driving means for the conveying rolls including a continuously operative frictional drive so constructed and arranged that when the sheet material shrinks, slippage occurs between the driving means and the rolls rather than between the rolls and the sheet material.

4. An apparatus for shrinking plastic sheet material containing internal stresses comprising an oven, means for heating the oven to a temperature at which the plastic material softens and releases stresses, means for conveying the sheet material through the oven including a series of closely spaced tubular rolls extending horizontally through the oven, and spreading means including tentering rolls spaced at intervals throughout the length of the oven, and

individual, continuously operative fluid driving means for the conveying rolls so constructed and arranged that when the sheet material shrinks, slippage occurs between the driving means and the rolls rather than between the rolls and the sheet material.

5. An apparatus fcr shrinking plastic sheet material containing internal stresses comprising an oven, means for heating the oven to a temperature at which the plastic material softens and releases stresses, means for supporting and conveying the sheet material through the oven including a series of closely spaced rolls, and driving means for the rolls including an individual fluid drive for each roll so constructed and arranged that when the sheet material shrinks, slippage occurs between thedriving means and the rolls rather than between the rolls and the sheet material.

6. An apparatus for shrinking plastic sheet material containing internal stresses comprising an oven, means for heating the oven, means for supporting and conveying the sheet material through the oven including tubular rolls, and driving and cooling means for the rolls comprising jets of air angularly directed against the inside walls of the rolls, said driving and cooling means being so constructed and arranged that when the sheet material shrinks, slippage occurs between the driving means and the rolls rather than between the rolls and the sheet material.

7. The method of shrinking plastic sheet material containing internal stresses which com prises conveying said sheet material through a heated area on hollow rolls driven and cooled by tangential jets of air impinging on the inside walls of the rolls.

HARVEY L. YQUNG. 

